1. Field of the Invention
This invention relates to a line switching method and a line switching apparatus for switching a plurality of multiplex transmission lines to select, for each of channels of transmission data transmitted in the multiplex transmission lines, one of the multiplex transmission lines in accordance with line alarm information in the transmission data of the channel.
2. Description of the Related Art
In such a time division multiplexed transmission system for digital signals as shown in FIG. 13, each node apparatus 200 detects a transmission trouble or an apparatus trouble to select a transmission line of a better transmission quality and thus switch the currently used line from a service line 201-1 to a protection line 201-2 or reversely, from the protection line 201-2 to the service line 201-1 in order to continue the service.
In order to switch lines for each channel of multiplexed transmission data, it is necessary to detect a pointer value of the channel or frame top and detect trouble information to supervise the lines in units of a channel.
FIG. 14 shows a construction of a line switching apparatus. Referring to FIG. 14, input data E (EASTs) SIDE DATA IN are inputted to a service line 201-1, and input data W (WEST) SIDE DATA IN are inputted to a protection line 201-2.
The line switching apparatus includes a DMUX section (demultiplexing section) 111 for demultiplexing serial data into parallel data of 8 bits, and another DMUX section 112 for demultiplexing multiplexed data into data in units of a channel. The DMUX sections 111 and 112 are provided for each of the service line 201-1 and the protection line 201-2.
The line switching apparatus further includes a channel processing section 113 provided for each of the E (EAST) side and the W (WEST) side. Each of the channel processing sections 113 includes a pointer value detection circuit 114 for a single channel, a top byte detection circuit 115 for the single channel, and an alarm detection circuit 116 for detecting various alarms (m alarms in the arrangement shown in FIG. 14) of the single channel. The line switching apparatus further includes a plurality of priority checking sections 117 provided for the individual channels for supervising m alarms on the E (EAST) side and m alarms on the W (WEST) side in a predetermined priority order to select, for each of the channels, a line having a better line quality, and a plurality of selectors 118 provided for the individual channels for switching, for each of the channels, the currently used line to a line having a better line quality.
The line switching apparatus further includes a MUX section (multiplexing section) 119 for converting demultiplexed data back into multiplexed data.
In the line switching apparatus of the construction described above, E side data of the service line 201-1 and W side data of the protection line 201-2 are expanded parallelly to data of the individual channels by the DMUX sections 111 and 112. The data of each channel are supplied to a corresponding one of the channel processing sections 113, in which a pointer is subsequently detected from the data by the pointer detection circuit 114 and then a top byte is detected by the top byte detection circuit 115, whereafter the m alarms are detected from the data by the alarm detection circuit 115.
Then, the E side data and the W side data are compared with each other for each of the corresponding channels by a corresponding one of the priority checking sections 117, and a corresponding one of the selectors 118 is switched in response to a result of the comparison by the priority checking section 118 to select a line having a better line quality. Thereafter, the demultiplexed data are converted back into original multiplexed data by the MUX section 119.
However, in line switching by the node apparatus described above, since multiplexed transmission data are demultiplexed in units of a channel and detection of a pointer, supervision for detection of alarms and switching of lines are processed parallelly for the individual channels, a plural number of processing sections, which have a same function, equal to the number of channels are required. This makes the scale of the apparatus large and requires a high cost accordingly.
Further, in the line switching apparatus described above, a large amount of a transmission delay results due to its parallel processing also, as can be seen from a STM-4 (STY: Synchronous Transfer Mode) shown in waveforms (a) to (i) of FIG. 15.
It is to be noted that reference characters (a) to (i) in FIG. 15 correspond to reference characters (a) to (i) in FIG. 14.
Further, since line switching at a low order group level wherein a large number of channels are involved cannot be performed by a high order group interface, the line switching apparatus has another problem to be solved in that expansion of the service to a general purpose apparatus cannot be achieved.